The invention is directed to an optical sensor, a method for producing an optical sensor, and also a method for detecting an object with an optical sensor.
Optical sensors for detecting objects can be constructed, for example, as diffuse-reflective sensors or as light barriers. They comprise a light source for transmitting visible or invisible light and a detector for receiving light, which is emitted by the light source. Light-emitting diodes, laser diodes, or IR diodes, for example, can be used as the light source. According to the construction of the sensor, the light source can be operated continuously or—for minimizing outside light influences—in a pulsed or clocked manner. It is also known to polarize the light and/or to focus the light using apertures and lenses or collimators to form a light beam.
As detectors, for example, phototransistors or photodiodes can be used. The light source and detector can be arranged, according to the purpose of the application and the function of the sensor, in a common housing or spatially separated from each other in separate housings.
For conventional diffuse-reflective sensors, the light emitted by the light source is usually focused by an aperture and a collimator lens to form a beam or a Gaussian beam with a nearly rotationally symmetric intensity distribution (relative to the propagation direction). Deviations from the rotational symmetry can be produced in LED light sources through their shape and in laser diodes based on the effects of refraction at their rectangular outlet opening. The focus or the narrow point of the light beam here determines the usable detection range. Conventionally, the beam diameter—this is defined by the converging of the beam diameter to the fraction 1/e radial to the beam direction—is usually kept as small as possible.
If this light beam strikes an object, it is at least partially reflected diffusely on its surface. A portion of the reflected light can be detected and evaluated by the detector. In other words: the light spot generated by the light beam on the object is imaged by the imaging optics arranged in front of the detector onto the light-sensitive surface of the detector.
Simple diffuse-reflective sensors merely evaluate the intensity of the captured light: the shorter the distance between the light source and measurement object, the higher the light intensity detected by the detector. By setting a switching threshold, a switching distance can be set for a certain type of measurement object.
Diffuse-reflective sensors with background masking and also distance sensors normally use the triangulation principle. In this way, the portion of light reflected by the object in the direction of the detector is imaged onto the detector and the position or location of the detected light on the detector is evaluated, with this position changing as a function of the distance between the sensor and object. The detector is constructed so that it can distinguish at least two different incident positions of the light reflected on a measurement object. As detectors, for example, two or more photodiodes or phototransistors, which are discrete components or which are integrated on a common substrate, can be used. Alternatively, detectors can also comprise one-dimensional or two-dimensional CCD arrays with high spatial resolution. By evaluating the difference in brightness on the individual pixels, the precise position of the main beam and from this the position of the detected object can be determined.
In conventional optical sensors, usually bulky, spherical glass or plastic lenses are used for influencing the light generated by a light source. These lenses are typically arranged between the generating light source and a front-side window that is transparent for the light of the light source, such that a focused light beam can be emitted with the smallest possible beam diameter. The lenses often require complicated alignment and/or holding devices and a lot of space. This is especially the case when several lenses are to be arranged one behind the other or one next to the other. The large space requirements set limits on the miniaturization of such sensors.
Conventional optical sensors are suitable not at all or only inadequately for detecting very thin or linear objects, such as, e.g., edges of films or other objects or even color marks, because such objects scatter only a small fraction of the light emitted by the light source so that the scattered light can be detected by the detector. The detection of lattice-like objects and objects with many small holes is also problematic with conventional sensors. In conventional sensors, the optics are tailored to a certain problem to be solved. Even slight changes to the initial conditions could make considerable adjustments necessary on the sensor housing, the holding device for the lens or lenses, and on the lenses themselves. In particular, the expense for aligning the light source, optics, and housing relative to each other is costly.